The invention relates to a linear lens system for rigid endoscopes for the conversion of a distal intermediate image, produced by an objective lens system, into a proximal intermediate image.
Rigid endoscopes are used in particular in medicine or technology for examining hollow spaces such as bodily cavities, engine combustion areas, power units, and the like. On the distal end an objective is arranged, which produces a first intermediate image. This intermediate image is converted by means of a downstream linear lens system into a proximal intermediate image plane, in which it can be observed with an eyepiece or a camera system. Depending on the depth of the hollow area to be examined, various conversion lengths are to be bridged by the linear lens system.
In the process of conversion, the linear lenses, also referred to as relay lenses, produce multiple image relays. The linear lens elements associated with an image relay here are designated as a relay system. An odd number of otherwise equal relay systems are usually presented. On the basis of cost and manufacturing considerations, the elements of the relay systems should be produced in the simplest possible way and should to a great extent be equal. The imaging scale should be 1:1.
Each individual relay system has imaging aberrations, which generally increase with the number of relay systems. To reduce imaging aberrations of the entire system, more complex relay systems, made up of several lens members in known manner, or lens members with varying refractive indexes, can be provided, as described for instance in DE 36 25 023 A1. Remaining imaging aberrations are usually compensated by a countervailing correction of the objective or the eyepiece.
From DE 38 54 156 T2, a single-use endoscope system of simple, economical construction is known, in which the arrangement of objective, conversion and observation lenses is made of a polymer material. The conversion system consists exclusively of a number of symmetric pairs of adjacent rod lenses, configured as identical biconvex lenses with refracting entry and exit surfaces of equal focal length. The objective lens system produces a first intermediate image on the distal entry surface of the conversion system. Additional intermediate images occur as relay images on the lens surfaces at the end of each pair of rod lenses. Out of cost considerations, it was decided to forego a correction of imaging aberrations, especially of the longitudinal chromatic aberration.
From U.S. Pat. No. 5,933,275, a linear lens system is known in which, going from the distal end, a number of relay systems is provided which each consist of two simple, biconvex linear lenses. The intermediate images occur between the relay systems in the air space. All linear lenses are separated from one another by a free space, where the intermediate images, however, are relatively close to the glass surfaces of the linear lenses.
On the proximal end are two biconvex linear lenses, each of whose ends turned toward one another has a negative meniscus lens cemented to it. The refractive index and the relative dispersion of the meniscus lens should be greater than with the linear lens part. In this proximal relay system consisting of cemented members, the longitudinal chromatic aberrations, arising as a whole from the simple relay systems mounted upstream, and the spherical aberrations should be compensated. In order to keep the spherical aberrations of the simple relay systems as small as possible, the linear lenses should be made of a glass with relatively high refractive index and low relative dispersion.
The image field convexity caused by the simple relay systems and the astigmatism should be compensated by a corresponding overcorrection of the objective.
In an additional embodiment, the proximal relay system is cemented to the ends turned toward each of the intermediate images together with a negative meniscus lens with a higher refractive index than the biconvex inner linear lens part. With the additional cemented member, the astigmatism caused by the simple relay systems should also be compensated by corresponding overcorrection.